In the combustion of a fuel, such as coal, oil, peat, waste, etc., in a combustion plant, such as a power plant, a hot process gas is generated, such a hot process gas, often referred to as a flue gas, containing, among other components, carbon dioxide, CO2. The negative environmental effects of releasing carbon dioxide to the atmosphere have been widely recognised, and have resulted in the development of processes adapted for removing carbon dioxide from the hot process gas generated in the combustion of the above mentioned fuels. One such system and process has previously been disclosed and is directed to a single-stage Chilled Ammonia based system and method for removal of carbon dioxide (CO2) from a post-combustion flue gas stream. Such a system and process has been proposed and taught in published US Patent Application 20080072762 (inventor: Eli Gal) entitled Ultra Cleaning of Combustion Gas Including the Removal of CO2, the disclosure of which is incorporated herein by reference.
FIG. 1A is a diagram generally depicting a flue gas processing system 15 for use in removing various pollutants from a flue gas stream FG emitted by the combustion chamber of a boiler system 26 used in a steam generator system of, for example, a power generation plant. This system includes a dust removal system 50 for removing dust/particulate matter (PM), a scrubber system 60 (wet or dry or a combination thereof) and a CO2 Removal system 70.
CO2 removal system 70 is configured to remove CO2 from the flue gas stream FG before emitting the cleaned flue gas stream to an exhaust stack 90. It is also configured to output CO2 removed form the flue gas stream FG. Details of CO2 Removal system 70 are generally depicted in FIG. 1B.
With reference to FIG. 1B, CO2 removal System 70 includes a capture system 72 for capturing CO2 from a flue gas stream FG and a regeneration system 74 for regenerating ionic solution used to remove CO2 from the flue gas stream FG. Details of capture system 72 are generally depicted in FIG. 1C.
With reference to FIG. 1C a capture system 72 of a CO2 capture system 70 (FIG. 1A) is generally depicted. In this system, the capture system 72 is a single-stage chilled ammonia based CO2 capture system. In a single-stage chilled ammonia based system/method for CO2 removal, an absorber vessel is provided in which an ionic solution is contacted with a flue gas stream containing CO2. The ionic solution may be composed of, for example, water and ammonium ions, bicarbonate ions, carbonate ions, and/or carbamate ions. An example of a known single stage CAP CO2 removal system is generally depicted in the diagram of FIG. 1C.
With reference to FIG. 1C, an absorber vessel 170 is configured to receive a flue gas stream (FG) originating from, for example, the combustion chamber of a fossil fuel fired boiler 26 (see FIG. 1A). It is also configured to receive a lean ionic solution supply from regeneration system 74 (see FIG. 1B). The lean ionic solution is introduced into the vessel 170 via a liquid distribution system 122 while the flue gas stream FG is also received by the absorber vessel 170 via flue gas inlet 76.
The ionic solution is put into contact with the flue gas stream via a gas-liquid contacting device (hereinafter, mass transfer device, MTD) 111 used for mass transfer and located in the absorber vessel 170 and within the path that the flue gas stream travels from its entrance via inlet 76 to the vessel exit 77. The gas-liquid contacting device 111 may be, for example, one or more commonly known structured or random packing materials, or a combination thereof.
Ionic solution sprayed from the spray head system 121 and/or 122 falls downward and onto/into the mass transfer device 111. The ionic solution cascades through the mass transfer device 111 and comes in contact with the flue gas stream FG that is rising upward (opposite the direction of the ionic solution) and through the mass transfer device 111.
Once contacted with the flue gas stream, the ionic solution acts to absorb CO2 from the flue gas stream, thus making the ionic solution “rich” with CO2 (rich solution). The rich ionic solution continues to flow downward through the mass transfer device and is then collected in the bottom 78 of the absorber vessel 170. The rich ionic solution is then regenerated via regenerator system 74 (see FIG. 1B) to release the CO2 absorbed by the ionic solution from the flue gas stream. The CO2 released from the ionic solution may then be output to storage or other predetermined uses/purposes. Once the CO2 is released from the ionic solution, the ionic solution is said to be “lean”. The lean ionic solution is then again ready to absorb CO2 from a flue gas stream and may be directed back to the liquid distribution system 122 whereby it is again introduced into the absorber vessel 170.
After the ionic solution is sprayed into the absorber vessel 170 via spray head system 122, it cascades downward onto and through the mass transfer device 111 where it is contacted with the flue gas stream FG. Upon contact with the flue gas stream the ionic solution reacts with CO2 that may be contained in the flue gas stream. This reaction is exothermic and as such results in the generation of heat in the absorber vessel 170. This heat can cause some of the ammonia contained in the ionic solution to change into a gas. The gaseous ammonia then, instead of migrating downward along with the liquid ionic solution, migrates upward through the absorber vessel 170, along with and as a part of the flue gas stream and, ultimately, escaping via the exit 77 of the absorber vessel 170. The loss of this ammonia from the system (ammonia slip) decreases the molar concentration of ammonia in the ionic solution. As the molar concentration of ammonia decreases, so does the R value (NH3-to-CO2 mole ratio). This decrease in the R value corresponds to a decrease in the effectiveness of the ionic solution in capturing CO2 from the flue gas stream.
The effectiveness of the capture system 72 in removing CO2 from a flue gas stream rests largely on: 1) the temperature (T) of the ionic solution sprayed into the absorber vessel 170, and 2) the mole ratio (R) of ammonia contained in the ionic solution to the CO2 contained in the ionic solution.
The general effect of R and Ton the systems CO2 capture efficiency is generally illustrated by the graph shown in FIG. 1D. The relative impact of ammonia slip is generally illustrated by the graph shown in FIG. 1E. In short, the lower the R value, the less effective an ammonia based CO2 capture system is in removing CO2 form a flue gas stream.
Temperature of the system may be controlled via heating and/or refrigeration systems. The mole ratio R of ammonia to CO2, however, can only be controlled by controlling the amount of ammonium in the ionic solution, since controlling the CO2 contained in the flue gas stream is not possible.
In order to minimize the amount of ammonia slip, the CO2 capture system 72 is preferably configured to operate at a low temperature (T), for example, a temperature from 0° C. up to 10° C. This may be achieved by, for example, controlling the temperature of the ionic solution introduced into the absorber vessel. It is also preferably configured to operate with the ionic solution having a low ammonia-to-CO2 mole ratio (R), for example, from 1.4 up to 1.8. This may be achieved by controlling the amount of lean solution introduced into the absorber vessel.
At low temperatures, for example, 0° C. up to 10° C., and low R values, for example, 1.4 up to 1.6 solid ammonium bicarbonate particles will precipitate from the ionic solution after it has been contacted with the flue gas stream. These solids contain very high concentrations of CO2 (approximately 55% by weight) that has been removed from the flue gas stream by virtue of the ionic solution being placed in contact therewith. Thus, the precipitation of the solids is desired since they contain high concentrations of CO2 and can be easily separated from the ionic solution and removed. However, in order to achieve the low temperatures required to cause solids to precipitate from the ionic solution, refrigeration equipment must be utilized. Additionally, in order to accommodate operation at a low R value, the volume/size of the absorber vessel 170 must be significantly increased.
The larger absorber vessel and refrigeration systems, as well as the operation thereof, are costly and greatly increase the costs associated with removing CO2 from a gas stream. A chilled ammonia based CO2 removal system having a single stage absorber system will thus be large, expensive and require high cooling capacity refrigeration systems to maintain a desired low operating temperature. Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.
Further, features of the present invention will be apparent from the description and the claims.